This invention relates to the desulfurization of gases and, in particular, relates to the microbiological disposal of hydrogen sulfide which has been otherwise removed from natural gas.
Natural gas from a well may contain a number of undesirable components which must be reduced to acceptable levels prior to distribution and sale. One of the most common problems in the gas industry is the removal and disposal of hydrogen sulfide. Hydrogen sulfide is an acid gas which is toxic and quite corrosive in the presence of water. Natural gas destined for the fuel market ordinarily must contain no more than 0.25 grains per 100 standard cubic feet or 4 ppm on a volume basis.
The most commercially important treatment system for the removal and disposal of hydrogen sulfide from natural gas consists of a combination of the amine process for removal from the gas stream followed by the Claus process for sulfur recovery. In the amine process, after contacting the gas stream, the amine solvent is heated to 200.degree.-300.degree. F. to liberate H.sub.2 S and regenerate the solvent which is recycled. It is important to note that the H.sub.2 S is removed from the gas stream but that it still must be disposed of. Hydrogen sulfide liberated during regeneration of the amine solvent is converted to elemental sulfur by the Claus process. In the Claus process, one third of the H.sub.2 S of the acid gas stream received from the amine unit is burned with a stoichiometric amount of air to produce sulfur dioxide according to Equation (1). If the entire acid gas stream is fed to the reaction furnace, some conversion of H.sub.2 S to elemental sulfur occurs in the furnace according to Equation (2). Further conversion is achieved by passing the reaction gas through a series of catalytic reactors where elemental sulfur formation proceeds more toward completion at lower temperatures. Alternately, one third of the acid gas stream may be fed to the reaction furnace for complete combustion of H.sub.2 S to SO.sub.2. The SO.sub.2 is then mixed with the remaining acid gases and fed to the catalytic reactors. EQU H.sub.2 S+3/2 O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O+heat (1) EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O+heat (2)
The Claus process produces a high quality elemental sulfur product and salvage heat value as process credits which have a significant positive impact on the economics of the process. However, there are inherent limitations and operating problems which may adversely affect the economics of the application of the process to H.sub.2 S disposal. These include the following:
(1) The maximum conversion efficiency with as many as three catalytic reactors in series is only 96-97%. Further treatment of the Claus tail gas may be required to meet local air quality standards.
(2) Conversion efficiency is sensitive to variations in the concentration of H.sub.2 S in the acid gas feed stream.
(3) In the presence of carbon dioxide (CO.sub.2) and light hydrocarbons, side reactions can result in the formation of carbonyl sulfide (COS) and carbon disulfide (CS.sub.2) in the reaction furnace. The presence of COS and CS.sub.2 may increase the number of catalytic stages requires for adequate H.sub.2 S conversion since COS and CS.sub.2 hydrolysis requires higher temperatures than those which favor conversion of H.sub.2 S to elemental sulfur according to Equation (2).
(4) At H.sub.2 S concentrations of less than 40% the temperature of the reaction furnace is insufficient to result in complete combustion of entrained hydrocarbons in the acid gas stream. Hydrocarbon reaction products can result in deactivation of the catalyst.
(5) Combustion of H.sub.2 S in the reaction furnace becomes more unstable with decreasing concentration of H.sub.2 S in the acid gas feed stream. At very low H.sub.2 S concentrations (less than 20%) preheating of air and acid gas streams is required. In addition SO.sub.2 must be generated by burning recycled elemental sulfur to ensure a proper stoichiometric H.sub.2 S/SO.sub.2 ratio in the feed to the catalytic reactors.
With sufficient H.sub.2 S available, a Claus plant can be profitable and offset other costs associated with natural gas treatment with sulfur sales and recovery of heat values. The break even point is influenced by those factors discussed above. However, because of increasingly stringent air quality standards for sulfur emissions, the Claus process has been applied in many treating situations where it is not economical. A need clearly exists for a new more economical technology in these situations especially with regard to acid gas streams with low concentrations of H.sub.2 S. A new technology which featured a saleable byproduct and greater conversion efficiency could also conceivably displace the Claus process in treating situations where it is presently regarded as economical. (Reference: Kohl, Arthur L. and Fred C. Riesenfeld, Gas Purification, Gulf Publishing Co., Houston, Tex., 3rd Ed., p. 410-421 (1979)).